Conventionally, a spark plug has been used to ignite an air-fuel mixture through spark discharge (may be referred to merely as “discharge”) for operation of an engine, such as an automotive internal combustion engine. In recent years, high output and low fuel consumption have been required of internal combustion engines. To fulfill such requirements, development of a plasma jet ignition plug has been conducted, since the plasma jet ignition plug provides quick propagation of combustion and exhibits such high ignition performance as to be capable of reliably igniting even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio.
The plasma jet ignition plug has a structure in which an insulator formed from ceramic or the like surrounds a spark discharge gap between a center electrode and a ground electrode, thereby forming a small-volume discharge space called a cavity. An example system of ignition of the plasma jet ignition plug is described. For ignition of an air-fuel mixture, first, high voltage is applied between the center electrode and the ground electrode, thereby generating spark discharge. By virtue of associated occurrence of dielectric breakdown, current can be applied between the center electrode and the ground electrode with a relatively low voltage. Thus, through transition of a discharge state from the spark discharge effected by further supply of energy, plasma is generated within the cavity. The generated plasma is jetted out through an opening (so-called orifice), thereby igniting the air-fuel mixture. For example, see Japanese Patent Application Laid-Open (kokai) No. 2006-294257 (Patent Document 1).
Meanwhile, the plasma jet ignition plug requires application of high-energy current during discharge. Application of high-energy current involves an increase in erosion of an electrode. Thus, in an attempt to restrain erosion of an electrode, a material having a high melting point is used to form the electrode. For example, see Japanese Patent Application Laid-Open (kokai) No. 2004-235040 (Patent Document 2). However, development of a plasma jet ignition plug which exhibits further restraint of electrode erosion and has high durability is awaited.